Low shrinkage silicone rubber composi-



United States Patent Ofitice 7 Re. 25,141 Reissued Mar. 27 1962 Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This invention is concerned with a filled organopolysiloxane convertible to the cured, solid, elastic state which shows a minimum of shrinkage during molding. More particularly, the invention relates to a heat-curable com-.

position of matter composed, by weight, of (l) 100 parts of a methyl vinylpolysiloxane convertible to the cured, solid, elastic state containing about 2.0 to 2.005 total methyl and vinyl groups, from about 0.05 to 2% of the silicon atoms being connected to at least one and not more than two vinyl radicals by a carbon-silicon linkage, (2) a finely divided mixture of silica fillers composed of (a) a precipitated silica having an ave-rage particle diameter of from 20 to 2 5 millimicrons and a surface area of about 14-0 to 160 square meters per gram, and (b) a diatomaceous earth having an average particle diameter of between 1000 to 6000 millimi-crons and having a surface area of about 15 to 30 square meters per gram, the total filler content comprising from 75 to 150 parts, the [diatomaceous earth] precipitated silica being present in an amount equal to from about 60 to 85% of the weight of the [precipitated silica] diatomaceous earth, the said filler content and proportions of filler varying with the durometer requirements of the cured product, and (3) from 0.5 to 2 parts bis-(2,4 dichlorobenzoyl) peroxide.

In the molding of certain silicone rubber products, particularly rings used as gaskets, etc., it has been found necessary, due to the large shrinkage (usually around 6% or more) of the usual silicone rubber during the molding technique to make allowances for any changes in tolerances in the dimensions of the gasket. Often this is very difiicult because the dimensional stability will vary with the type of polymer, the type of filler, the mold, etc. Because of this variation, it has been almost impossible to employ for molding silicone rubber, molds which have been used in the past for molding other synthetic rubber such as hydrocarbon rubbers, for instance, copolyrners of butadiene and styrene, copolymers of butadiene and acrylonitrile, natural rubber, etc., whichhave linear shrinkages of the order of about 1.6 to 1.8%. This, of course, is undesirable because scraplosses, as well as the difficulty in taking into account any changes in dimensions of the molded product from the time it is put into the mold until the time it is finally molded. In addition, special molds have had to be prepared for molding silicone rubber since molds which may have been made for use in the usual rubber molding art have been unacceptable.

Unexpectedly, I have discovered that a specific combination of ingredients employing a special mixture of fillers and a certain curing agent in combination with a methyl vinyl gum convertible to the cured, solid, elastic state can be employed as a molding composition which, when molded in the usual fashion, result in a product whose linear shrinkage is essentially equivalent to the linear shrinkage of previously known organic rubbers, particularly the hydrocarbon type of rubbers mentioned above, that is, those other than silicone rubbers. These compositions have been so uniform in their low shrinkage 2 that in the molding about 2,000 O-rings from the above described filled methyl vinylpolysiloxane, every molded sample had the same dimensions as 2,000 other samples molded in the same mold using a butadiene-acrylonitrile copolymer.

The o'rganopolysiloxane gum convertible to the cured, solid, elastic state employed in the practice of my invention is a methyl vinylpolysiloxane containing about 2 to 2.005 total methyl and vinyl groups, from 0.05 to 2% of the silicon atoms of the organopolysiloxane being sub stituted with vinyl radicals. Examplesof suchv methyl vinyl polysiloxaneswhich may be-employed as well as methods" for preparing the same are described in Ma-rsden Patent 2,445,794, which by reference is made part of the present disclosures. More specifically, such heat-eonvertible methyl vinylpolysiloxane can be obtained by inte'rcond'ensing octamethylcyclotetrasiloxane with tetramethyl tetra'vinylcyclotetrasiloxane in such proportions that the number of vinyl groups on 'silicon' iswithin the range described above. Such interpolymerization can be effected by means of small concentrations of potassium hydroxide, cesium hydroxide, etc. The amount of inter polymerization catalyst used is relatively small and, depending on the type of interpolymerization catalyst used, may range from 0.001 to 0.1%, by weight, based on the total weight of the polys'iloxanes.

The particular'combination of fillers employedincludes a precipitated silica having. an average particle diameter of about 20 to 25 millimicrons and a surface area of about to square meters per gram. Such a materialis Hi-Sil X3 03,, a precipitated silica manufactured by Columbia-Southern Chemical Corporation, Barberton, Ohio. It is generally prepared by adjusting the pH of an aqueous solution of sodium silicate and precipitating the silica with an acid, e.g., hydrochloric acid. This finely divided filler has a pH of. from about 7.0 to 8.0.

The other finely divided silica filler employed is of somewhat larger average particle size diameter and ranges from about 1,000 to 6,000 rnillimicrons. This material has a pH of about 7.0 to 8.5. An example of such a silica is diatomaceous earth sold as Celite 270, Celite 350, Celite Super-floss, etc.,'by Johns-Manville Products Corporation, Maplewood and Craft Streets, Albany, New York.

The catalyst specifically employed in-the practice of the present invention which has been found to give optimum shrinkage characteristics is bis-(2,4-dichlorobenzoyl) peroxide. It is important that this catalyst be employed and is preferably present, by weight, in an amount equal to from 0.5 to 2 parts of the latter per 100 parts of the convertible methyl vinylpolysiloxane. In general, this is the only catalyst or curing agent for the methyl vinylpolysiloxane required to effect curing of the latter, and is satisfactory for most stocks of cured silicone rubber over a wide range of durometers ranging from about 50 to 80 on the Shore durometer \A scale. It has been found that is the higher durometer range, for instance, durometers at around 80, the bis-(2,4-dichlorobenzoyl) peroxide is the only catalyst required and the desired durometer hardness will be obtained provided the molding composition is molded shortly after the ingredients are mixed together. If the mixture of ingredients is stored for any length of time, for instance, from several days to several months, it will be found that there is a tendency for the durometer of the molded products to drift downward and decrease with increased storage of the moldable material. Thus, whereas a durometer of 80 can be obtained if the molding is carried out promptly after mixing the ingredients, if the moldable mixture of ingredients is allowed to stand for eight to ten days, it will be found that the durometer of" the molded product maybe 75 or even as low as 65. In order to avoid this undesirable change in durometer hardness, I have found'that the incorporation of another curing agent, specifically benzoyl peroxide, in an amount equal to from 0.05 to 1 part thereof per 100 parts of the convertible methyl vinylpolysiloxane, in combination with bis-(2,4-dichlorobenzoyl) peroxide, markedly reduces and in some-instances cornpletelyeliminates this tendency to change in durometer. In order to preparethemoldable compositions, it is only necessary to mix the ingredients together described above with the bis-(2,4-dichlorobenzoyl)peroxide on the usual rubber compounding mill, and thereafter molding the same at a temperature of about 140 to 200 C. for .tirnes .ranging from about 5 to 30 minutes or more. After the molding operation, it is usually desirable to fur- {ther heat-age the molded product at a temperature, for instance, from'150 to 300 C. for times ranging from a few minutes to as long as 24 hours or more to obtain the ultimate cure.

For minimum shrinkage (e.g., from 1.5 to 2.0% linear shrinkage) itis essential that all volatile materials boiling below 200 C. (when measured at 760 millimicrons) be removed from the methyl vinylpolysiloxane convertible to the cured, solid, elastic state. This can be accomplished by heating the latter at a temperature of about 100 to 200 0., preferably under reduced pressure prior to incorporation therein of the fillers and peroxide. Means for accomplishing this removal of volatiles (which may consist of low molecular weight cyclic polydimethylsiloxanes) are found disclosed and claimed in Hatch and Blumenfeld application Serial No. 396,068, filed Decemher 3, .1953, and assigned to the same assignee as the present invention.

The above molded compositions have linear shrinkages of about 1.5 to 2.0%, while the usual silicone rubbers now commercially available exhibit linear shrinkages ranging from about 3 to as high as 6 to 10% or more.

In order that those skilled in the art may better understand how the present invention may be practiced, the following examples are given by way of illustration and not by way of limitation. All parts are by weight.

The percent linear shrinkages were determined by measuring the width and length of the mold cavity, taking the average of the two dimensions (in millimeters), and then comparing the average dimensions of the heataged (24 hours at 250 C.) molded sheet with the average dimension of the mold cavity. The formula used is 11+12 'r-H'z T 2 Percent linear shrinkage= X 100 100 parts octamethylcyclotetrasiloxane were mixed with 0.23 part of a hydrolyzate of methyl vinyldichlorosilane, which was composed of mixtures of cyclic methyl vinylpolysiloxanes of the formula [(C H (CH )SiO],, where n is a whole number equal to from 3 to 6. To this mixture was aded about 0.001 part potassium hydroxide, and the mixture of ingredients was heated at about 150 to 170 C. for about one hour to obtain a highly viscous polymer composed of intercondensed dimethylsiloxy units and methyl vinylsiloxy units. This polymer was then washed with water (in the ratio of about 100 parts polymer to 1 0 parts water) on a doughmixer to remove essentially all the potassium hydroxide employed as catalyst. Thereafter the polymer was heated with water at a temperature of around 175 to 200 C. for several hours so as to steam strip the low boiling volatiles, until there was less than 0.3 to 1% volatiles when a one-gram sample thereof was heated for 30 minutes under vacuum of less 4 than 5 mm. at C. This low volatile-containing poly mer was identified as vinyl methylpolysiloxane.

EXAMPLE 2 100 parts of the methyl vinylpolysiloxane described in Example 1 were mixed with 52 parts of Hi-Sil X3 03 and 85 parts diatomaceous earth (Celite 350), both of which are more particularly described above to provide about 61% by weight precipitated silica, based on the weight of the diatomaceous earth. To this mixture of ingredients were added 10.68 part bis-(2,4-dichlorobenzoyl) peroxide and 0.2 part benzoyl peroxide. Other formulations were prepared employing the I-Ii-Sil X303 alone with the same devolatilized methyl vinylpolysiloxane and, in one instance, there was also employed a non-vinyl-containing methylpolysiloxane obtained by condensing octamethyl cyclotetrasiloxane with KOH, and removing the KOH and devolatilizing the gum similarly asdescribed above. Each of the samples was molded at about to C. for 15 minutes into the form of flat sheets, and thereafter the flat molded samples were further heated at 250 C. for 24 hours in an air-circulating oven, after which the linear shrinkages were determined as described above. The following Table I shows the formulations employed in each instance, as well as the percent linear shrinkage of each molded sample, and the physical properties of the sample coming within the scope of the present invention (sample No. 1).

Table I Sample No. Ingredients (Parts) Devolatillzed methyl vlnylpolysiloxane. 100 100 Devolatilized methylpoly 100 100 X303 52 52 52 52 Celite 350 85 85 Bis-(2,4-dichlorobenzoyl) peroxide 0, 68 0.6 0. 6 0. 68 Benzoyl peroxide 0. 2 0. 2 0. 2 0. 2 Properties: 3

Linear shrinkage (percent) 1. 67 2. 87 2. 67 2. 27 Tensile Strength p s i 1, 300 Percent elon ation 70 Percent compression set (22 hours at I so Durometer. I After 24 hours heat-aging.

EXAMPLE 3 In this example two formulations of different durometers were prepared employing the methyl vinylpolysilox anc gum described in Example 1. Each sample was molded and heat-treated as described in Example 2. The

following Table II shows the formulations used as well I as the properties of the molded products after the 250 C. heat aging. In sample No. 5 below, the precipitated silica was present in an amount equal to about 82% by weight of the diatomaceous earth and in sample No. 6 the precipitated silica was present in an amount equal to about 59% on this same basis.

It will, of course, be apparent to those skilled in the art that other minor variations in proportions of the ingredients described in the above-identified example may be employed without departing from the scope of the invention. It is critical that the combination of the two types of silica fillers of the specified particle size as well as the particular curing agent be employed in the narrow proportions recited if one is to obtain the above-described unexpected results. As is evident from an examination of Example 2, the omission of one of the fillers or the use of a convertible methylpolysiloxane free of silicon-bonded vinyl groups will not give the results described above.

In addition to the manufacture of gaskets which can be used in applications requiring close tolerances (and where resistance to lubricating oils and hydraulic fluids is desired), the compositions herein described can also be employed in making molded products such as enclosures or boots for various types of equipment where close tolerances are required and where it is desired to avoid the expense of making separate molds for the molding of the silicone rubber compositions in order to obtain those close tolerances.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A composition of matter exhibiting during molding low linear shrinkage comprising, by Weight, (1) 100 parts of a methyl vinylpolysiloxane convertible to the cured, solid, elastic state containing about 2.0 to 2.005 total methyl and vinyl groups, from about 0.05 to 2% of the silicon atoms being connected to at least one and not more than two vinyl radicals by a carbon-silicon linkage, (2) a finely divided mixture of silica fillers composed of (a) a precipitated silica having an average particle diameter of from 20 to 25 millimicrons and a surface area of about 140 to 160 square meters per gram, and (b) a diatomaceous earth having an average particle diameter of between 1,000 to 6,000 millimicrons and having a surface area of about 15 to 30 square meters per gram, the total filler content comprising from to 150 parts, the [diatomaceous earth] precipitated silica being present, by weight, in an amount equal to from about 60 to of the weight of the [precipitated silica] diatomaceous earth, the said filler content and proportions of fillers varying with the hardness requirements of the cured product, and (3) from 0.5 to 2 parts bis-(2,4-dichlorobenzoyl) peroxide.

2. A composition of matter exhibiting during molding low linear shrinkage comprising, by weight, (1) parts of a methyl vinylpolysiloxane convertible to the cured, solid, elastic state containing about 2.0 to 2.005 total methyl and vinyl groups, from about 0.05 to 2% of the silicon atoms being connected to at least one and not more than two vinyl radicals by a carbon-silicon linkage, (2) a finely divided mixture of silica fillers composed of (a) a precipitated silica having an average particle diameter of from 20 to 25 millimicrons and a surface area of about to 160 square meters per gram, and (b) a diatomaceous earth having an average particle diameter of between 1,000 to 6,000 millimicrons and having a surface area of about 15 to 30 square meters per gram, the total filler content comprising from 75 to parts, the [diatomaceous earth] precipitated silica being present, by weight, in an amount equal to from about 60 to 85% of the weight of the [precipitated silica] diatomaceous earth, the said filler content and proportions of fillers varying with the hardness requirements of the cured product, (3) from 0.5 to 2 parts bis-(2,4-dichlorobenzoyl) peroxide, and (4) from 0.05 to 1 part benzoyl peroxide.

References Cited in the file of this patent or the original patent UNITED STATES PATENTS 2,666,041 Pfeifer Jan. 12, 1954 2,723,966 Youngs Nov. 15, 1955 2,816,089 Willis Dec. 10, 1957 

